Magnets serve an indispensable function in our technology-based society and are ubiquitous in all varieties of mechanical and electronic devices in science and industry. Traditional magnets are atom-based, and are comprised of the transition, lanthanide, or actinide metals, with the magnetism arising from the magnetic dipole moment that is a product of the presence of unpaired electrons in the d- or f-orbitals.
Previous research attempts to design and synthesize molecular organic magnets and high-spin molecules with intrinsic magnetic properties were unsuccessful and very few have been found to be of industrial use, as such molecules have a fairly low ferromagnetic transition temperature, commonly referred to as Curie temperature (Tc). There remain fundamental obstacles that seem to block the ability to resolve scientific difficulties in developing organic magnets with high Tc (much higher than room temperature). There are only a few examples of organic magnets that have Tc above room temperature, but such materials are insoluble in common solvents (e.g., toluene, acetone, and tetrahydrofuran) and infusible as well as unstable under ambient environment. Thus the problem of fabrication of magnetic films and liquid magnets still remains unresolved. Since the magnetic anisotropy in organometallic magnets is considerably lower than that in the case of metal-containing compounds arising from the weak spin-orbital coupling between s and p electrons, high-Tc molecular magnets have not yet been realized.
Development of molecule-based magnetic polymers would be worthwhile because they may exhibit numerous desirable properties, including solubility, processability, and synthetic tenability. Such features are a direct result of the molecular nature of molecule-based magnetic polymers and are not shared by traditional atom-based magnets. Molecule-based magnetic polymers provide prospects for new nanoscale molecular materials as functional magnetic memory devices leading to dramatically enhanced data processing speeds and storage capacity in computers or many other applications. Such polymeric magnets would be lighter, more flexible, and less intensive to manufacture than conventional metal and ceramic magnets. Just for example, applications could include magnetic shielding, magneto-optical switching, and candidates for high-density optical data storage systems.
The theory of magnetism is primarily based on two quantum mechanical concepts: electron spin and the Pauli Exclusion principle. From the Curie law, the magnetic susceptibility (χ) is expressed by χ=N2gβ2S(S+1)/3kBT where β is the effective magnetic moment, g is g-factor, N is Avogadro's number, S is the spin angular momentum, kB is the Boltzmann constant, and T is the absolute temperature. Thus, χ is proportional to S2 (thus high spin is required for high magnetic properties), but inversely proportional to T. Also, there is a critical temperature, Tc, below which the ferromagnetic materials exhibit spontaneous magnetization. To date, the most challenging issue for the synthesis of molecule-based magnetic polymers is to increase the Tc to well above room temperature, which is desirable for industrial applications.
The conventional molecular/organic magnets used at present are all atom-based. They exist in the form of crystals or complexes through noncovalent bonds (e.g., hydrogen bonding, ionic interactions, or metal coordinations), and thus spin coupling largely depends on the lattice distance of the crystal, because the exchange interaction is proportional to 1/r10, wherein r is the bond length. Some efforts have been directed to the formation of a charge-transfer (CT) complex to design and synthesize molecular/organic magnets. It has been noted that there are positive and negative spin densities in certain structures (e.g. aromatic radicals), and that atoms of positive spin density are exchange coupled most strongly to atoms of negative spin density in neighboring molecules. The delocalization of spin density in macromolecular chains makes it possible for magnetic interactions to take place across extended bridges between magnetic centers separated from each other, propagating through conjugated bond linkages, which act as molecular wires. Spin polarization (i.e., the simultaneous existence of positive and negative spin densities at different locations within a given radical) is needed for intermolecular exchange interactions to bring about ferromagnetic interactions. Employing iron or transition metal with larger radial orbitals as magnetic centers will improve the overlap between the orbitals of electron acceptor (A−) and electron donor (D+), namely spin coupling. Currently, there have been no successful attempts reported on the synthesis of molecule-based donor-acceptor magnetic polymers.
Magnetic polymers based on p-orbital spins typically exhibit weak ferromagnetic properties and thus Tc is still below 10 K even when S reaches 5000. Therefore, it is necessary to incorporate much stronger magnetic centers into the macromolecular chains, such as iron or other transition metals having the unpaired electrons located in d- or f-orbitals.
Existing superparamagnetic nanocomposites typically contain magnetic particles (e.g., Fe, Co, Ni etc.) in the form of powder or flakes in a non-magnetic polymer matrix. Due to the tendency of magnetic particles to aggregate when added to a non-magnetic polymer matrix, the magnetic particles are typically treated with a surfactant or another polymer in order to suppress aggregation. Because magnetic particles have a much higher density than the non-metallic polymer matrix, the magnetic particles tend to settle out at rest or during storage. This results in non-uniform dispersion of magnetic particles in the polymer matrix and poor heat dissipation during use.
The volume fraction of the magnetic particles in superparamagnetic nanocomposites is much smaller than that of the matrix polymer, and therefore the resulting magnetic level is not high. Thus, applications of superparamagnetic nanocomposites are limited.
Superparamagnetic nanocomposites are further limited by their lack of solubility in common solvents. This prevents them from being used in the preparation of intrinsically homogeneous magnetic fluids (liquid magnets). Thus, magnetic particles (e.g., iron oxide or ferrite) are suspended in a carrier liquid to prepare so-called ferrofluids, and they are used in industry. Ferrofluids are characterized as suspensions of magnetic particles in a carrier fluid, and they suffer from the same problem as superparamagnetic nanocomposites in that the magnetic particles tend to aggregate and also sediment at rest.
Ferrofluids currently in use are typically suspensions containing magnetic particles (iron oxide or ferrite for example) with typical volume fractions of 0.3-0.4 in a carrier fluid (typically silicone oil). There is another type of suspensions of magnetic particles, referred to as magnetorheological fluid (MR) fluid. The difference between ferrofluids and MR fluids lies in the size of magnetic particles. Whereas the sizes of magnetic particles used to prepare ferrofluids are about 5-20 nanometers (nm), the sizes of the magnetic particles used to prepared MR fluids are about 5-20 micrometers (μm), i.e., about 1000 times larger the particle size normally used for the preparation of ferrofluids. The conventional, commercially available MR fluids typically contain an organic additive in order to stabilize the dispersion of aggregates of magnetic particles. Due to the large difference in density between the magnetic particles (having a density of 5-6 g/cm3) and a typical carrier fluid (having a density less than 1 g/cm3), the conventional MR fluids have serious technical problems. In particular, the magnetic particles in the conventional MR fluids settle out over a relatively short period of time (i.e., in a few minutes to a few hours). Another technical difficulty is related to the lack of redispersibility of the magnetic particles in the conventional MR fluids. After the magnetic particles settle, they form highly dense aggregates, the extent of which depends on the chemical structure of a carrier fluid. To help disperse the aggregates of magnetic particles in a heterogeneous MR fluid, considerable efforts have been spent on treating the particles with a surfactant or a polymeric gel during the preparation of such MR fluids, but these attempts have not resolved the deficiencies.
Notwithstanding the state of the art as described herein, there is a need for further improvements in molecule-based (i.e., homogeneous) magnetic fluids and polymers. These types of fluids and polymers (without the presence of magnetic nanoparticles) would have numerous applications and would enable the preparation of intrinsically homogeneous liquid magnets without the need for magnetic particles, which can then replace ferrofluids or MR fluids that have inherent difficulties of sedimentation and aggregation of magnetic particles, and other deficiencies.
It would be worthwhile to have molecule-based magnetic polymers that may alleviate these deficiencies.